An 82 year-old male with multiple myeloma comes to the emergency room with fatigue and weakness. His hemoglobin is 6.5 g/dL. He receives a blood transfusion and within 30 minutes after the transfusion is started, he develops burning at the infusion site, fever,chills, chest tightness, backache, headache, flank pain and reddish-colored urine. His temperature is 101 ⁰ F, heart rate is 94 beats per minute, respiratory rate is 26 breaths per minute, and blood pressure is 80/55 mm Hg. On physical examination, you see flushing on his face and hear wheezing in the lungs. Laboratory tests reveal hemoglobin 5.9 g/dL, free hemoglobin in urine, decreased serum haptoglobin, increased lactate dehydrogenase, increased indirect bilirubin level and positive direct antigen (Coombs) test. What is the underlying mechanism of this disorder?
